Toward a more robust lower neck compressive injury tolerance-an approach combining multiple test methodologies.

OBJECTIVE The compressive tolerance of the cervical spine has traditionally been reported in terms of axial force at failure. Previous studies suggest that axial compressive force at failure is particularly sensitive to the alignment of the cervical vertebra and the end conditions of the test methodology used. The present study was designed to develop a methodology to combine the data of previous experiments into a diverse data set utilizing multiple test methods to allow for the evaluation of the robustness of current and proposed eccentricity based injury criteria. METHODS Data were combined from 2 studies composed of dynamic experiments including whole cervical spine and head kinematics that utilized different test methodologies with known end conditions, spinal posture, injury outcomes, and measured kinetics at the base of the neck. Loads were transformed to the center of the C7-T1 intervertebral disc and the eccentricity of the sagittal plane resultant force relative to the center of the disc was calculated. The correlation between sagittal plane resultant force and eccentricity at failure was evaluated and compared to the correlation between axial force and sagittal plane moment and axial force alone. RESULTS Accounting for the eccentricity of the failure loads decreased the scatter in the failure data when compared to the linear combination of axial force and sagittal plane moment and axial force alone. A correlation between axial load and sagittal plane flexion moment at failure (R² = 0.44) was identified. The sagittal plane extension moment at failure did not have an identified correlation with the compressive failure load for the tests evaluated in this data set (R² = 0.001). The coefficients of determination for the linear combinations of sagittal plane resultant force with anterior and posterior eccentricity are 0.56 and 0.29, respectively. These correlations are an improvement compared to the combination of axial force and sagittal plane moment. CONCLUSIONS Results using the outlined approach indicate that the combination of lower neck sagittal plane resultant force and the anterior-posterior eccentricity at which the load is applied generally correlate with the type of cervical damage identified. These results show promise at better defining the tolerance for compressive cervical fractures in male postmortem human subjects (PMHS) than axial force alone. The current analysis requires expansion to include more tolerance data so the robustness of the approach across various applied loading vectors and cervical postures can be evaluated.